Gas-Liquid Chromatographic Hydrogenolysis of Furan Derivatives D. A. George and L. M. McDonough Entomology Research Division, Agriculture Research Service, U S D A , Yakima, Wash. 98902
IN 1960, THOMPSON et af. ( I ) reported a technique for structural analysis in which organic compounds were volatilized at temperatures of 150-350°C in a stream of hydrogen and passed over a hydrogenation catalyst. Multiple bonds were saturated and functional groups were removed so that a saturated hydrocarbon was produced with the original carbon skeleton intact except that with some functional groups, the connected carbon atom was also removed. Beroza et al. ( 2 ) then developed a catalyst tube which was connected to a gas chromatograph and allowed hydrogenolysis and detection of the product in one step. Our interest in this technique arose from our need to determine whether natural products containing a furan ring were 2-alkyl or 2,Sdialkyl substituted. Since furan derivatives suitable for testing are not commercially available, we synthesized two such compounds and determined their hydrogenolysis products. EXPERIMENTAL
Hydrogenolysis was conducted with the National Instrument Laboratories Beroza Carbon Skeleton Determinator. The catalyst was neutral 1 Pd on Gas-Chrom P and the hydrogenolysis temperature was 260 “C. 2-Hexyl-5-methylfuran was prepared from 1-bromohexane and 2-methylfuran ( 3 ) ; retention index on Apiezon-L = (1) C. J. Thompson, H. J. Coleman, R. L. Hopkins, and H. T. Rall, J. Gas Chromatogr., 5,1 (1967) and references therein. (2) M. Beroza and F. Acree, J. Ass. Offic.Agr. Chem., 41, 1 (1964) and references therein.
1175); n ~ 2 *= 1.4476. 2-(l-Heptenyl)furan was prepared by a Wittig reaction from furfural and I-bromohexane. Proof of structure was by MS and IR; retention index on SE-30 = 1270; n~~~= 1.4928. RESULTS AND DISCUSSION
2-Hexyl-S-methylfuran, upon hydrogenolysis, gave undecane and no decane; 2-(l-heptenyl)furan gave decane and no undecane. Thompson et a f . found that 2,s-dimethylfuran gave hexane and pentane. Pentane is an anomalous product in that it was produced by the removal of a carbon atom not connected to the functional group. Here Thompson et al. used a catalyst of palladium-on-alumina. Thompson et a f . also noted loss of methyl for C-methyl substituted carbazoles with a catalyst of platinum-on-glass. With the catalyst we used, our two examples indicate that alkyl substituted furans will undergo hydrogenolysis normally and a monosubstituted furan will lose a carbon atom, but a disubstituted furan will not. RECEIVED for review December 9, 1971. Accepted February 18, 1972. The mention of a proprietary product in this paper does not constitute a recommendation or an endorsement of this product by the US.Department of Agriculture. (3) Office de Recherches Industrielles de Laboratoire, French Patent No. 1,186,346; Chem. Absfr.,56, 455a (1962).
Determination of Active Hydrogen in Organic Compounds by Chemical Ionization Mass Spectrometry Donald F. Hunt,’ Charles N. McEwen, and R. A. Upham Department of Chemistry, University of Virginia, Charlottesville, Vu. 22901
STRUCTURE ELUCIDATION of a complex natural product is often aided considerably by knowledge of the number of acidic hydrogens (i.e. 0 - H , N-H, S-H and CO-OH) present in the molecule. This is particularly true if the heteroatom content has already been delineated by high resolution mass spectrometry. On a microgram scale, the qualitative determination of active hydrogen is best accomplished by equilibrating the sample with DzO or CHIOD and using mass spectrometry to measure the resulting change in molecular weight (I, 2). 1
Author to whom correspondence should be addressed.
(1) K . Biemann, “Mass Spectrometry,” McGraw-Hill, New York, N.Y., 1962, Chap. 5. (2) H. Budzikiewicz, C. Djerassi, and D. H. Williams, “Structure Elucidation of Natural Products by Mass Spectrometry,” Vol. I, Holden-Day, Inc., San Francisco, Calif., 1964, Chap. 2. 1292
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
Previously this type of analysis has been performed by adding a slurry of the sample in deuterium oxide directly to the inlet system of a mass spectrometer (3) or by allowing the sample to stand in either D20 or CH30D, evaporating the solvent, and then injecting the sample directly into the ion source of the mass spectrometer ( I , 2). When either of the above methods is employed in conjuction with a mass spectrometer operating in the conventional electron impact (EI) mode, several problems are often encountered. These include loss of deuterium due to exchange between sample and water absorbed on the walls of the ion-source and inlet system of the mass spectrometer; loss of sample during minipulations involved in the isotope exchange step; and failure of many organic compounds to form stable molecular ions when they are ionized by electron impact. (3) J. S . Shannon, A u s f .J . Chem., 15,265 (1962).
'O01
62
OH
@
75-
50
-
H
42 25-
$ 4
2
2
82 I
4 100-
ketoestradiol (top); CI(DzO)mass spectrum of adenosine (bottom)
274
w >
F
5
8
75-
50-
42
62
OH
OH
'
270
290
m IC
As part of a continuing effort to develop chemical ionization mass spectrometry (CIMS) for structure elucidation work, we have recently examined the utility of water as a reagent gas. In this report we describe a convenient procedure for the determination of active hydrogen by CIMS using D O as the reagent. EXPERIMENTAL
All spectra were recorded on an Associated Electronics Industries MS-902 mass spectrometer equipped with a dual EIjCI source manufactured by Scientific Research Instruments Corporation (Baltimore, Md.) (4). Operating conditions were as follows: electron energy, 500 eV; accelerating potential, 8 kV; ion source temperature, 150-200 OC; reagent gas (D2O) pressure, 0.4 Torr; resolution, 3000. Samples were introduced by injection through a rubber septum or by vaporization from a heated solid probe. Following experiments with D20,it is necessary to purge the mass spectrometer with water (0.4 Torr) for 10-15 min before satisfactory spectra can be obtained with nondeuterated reagent gases. Routine use of water as a reagent gas at 0.4 Torr had no adverse effect on the performance of the mass spectrometer.
+ DzO
DzO*+ D30+
---f
D30+
+ *OD
+ n(D20) -+ D+ (D20)n+l n = 1, 2, 3, 4
as Bronsted acids in the gas phase and deuterate many organic compounds. In addition as a consequence of the relatively high source pressure (0.4 Torr), sufficient collisions occur between sample and D20 to exchange all of the hydrogens bonded to oxygen, sulfur, and nitrogen atoms for deuterium in the organic molecule. Two examples which illustrate the above method are shown in Figure 1. The most abundant ion in the CI(D20) spectrum of the nucleoside, adenosine (l),(Figure 1 bottom) occurs at m/e 274 and corresponds to ds-adenosine D+. Since this same ion appears at m/e 268 in the CI(H10) spectrum, the above result clearly indicates that all five acidic hydrogens in the nucleoside suffer exchange in the ion source when DzO is employed as the reagent gas. In addition, two fragment ions resulting from cleavage of the glycosidic linkage are also observed in analogy to CI(CH4) (7) and E1 (8) spectra. These appear at m/e 136 and 140 for deuterated 1 and correspond to the d3-sugar moiety 2 and d3-adenine D+ 3.
+
+
RESULTS AND DISCUSSION In the CI technique (5, 6), a set of ions is generated by bombarding a reagent gas with high energy electrons (70-500 eV) at a pressure of ca. 1 Torr. Sample molecules are introduced into the mass spectrometer by the usual methods and are ionized by ion-molecule reactions with the reagent ions. At a pressure of 0.4 Torr, electron bombardment of DzO affords abundant ions at m/e 22(D30+), 42(D602+), 62(D@3+), 82(D904+), and 102(D110a+). These ions, in turn, function
3
2
D2O " ;DzO.+
In the CI(D20) spectrum of 6-ketoestradiol (4) (Figure 1 top) the M 1 peak observed in the methane and water CI spectra at m/e 287 is shifted to mje 290. This latter ion corresponds to d2-4 D+ and results from exchange of the two active hydrogen atoms in 4 followed by deuteration.
(4) D. Beggs, M. L. Vestal, H. M. Fales, and G. W. A. Milne, Rev. Sci. Instrum., 42, 244 (1971). ( 5 ) F. H. Field, Accourzrs Chem. Res., 1,42 (1968). (6) M. S. B. Munson, ANAL.CHEM., 43(13), 28A (1971).
(7) M. S. Wilson, I. Dzidic, and J. A. McCloskey, Biochim. Biophp. Acta., 240, 623 (1971). (8) K. Biemann and J. A. McCloskey, J. Amer. Chem. SOC.,84, 2005 (1962).
+
+
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
1293
Elimination of DzO from deuterated 4 affords the abundant ion at m/e 270. As expected only one molecule of DzO is lost since the second hydroxyl group is attached to the aromatic ring. In contrast, the cardenolide, digitoxigenin (5), affords a CI(D20) spectrum containing peaks corresponding to the successive loss of DzO and HDO from the d2-5 D+ion.
+
OH
. I
4
5
In addition to the above examples, we have also obtained CI (D20) spectra on a number of compounds containing one or more common organic functional groups. Our findings
indicate that hydrogen bonded to heteroatoms in alcohols, phenols, carboxylic acids, amines, amides, and mercaptans undergoes essentially complete exchange in the ion source when DzO is employed as the reagent gas at a pressure of 0.4 Torr. Small amounts of deuterium incorporation (
